I joined NSCS thinking "resume fodder" but by the time I applied to medical school I had a lot more interesting, worthwhile, substantial experiences that made it onto the AMCAS instead. That's not to say that the mentoring and stuff I was able to do through NSCS wasn't worthwhile, but that it was shallow by comparison to most other activities. So I say, join it if you want to, but make it a goal to do so many other meaningful things that you ultimately don't even mention your membership in it when it comes time to apply.